Explained: Bandgap

Jul 23, 2010 by David L. Chandler

Why do some materials work well for making solar cells or light-emitting diodes (LEDs), while other materials don't? One key factor is having the right bandgap.

In a nutshell, bandgaps have to do with how behave and what it takes to get them excited. Electrons are the subatomic particles that carry a negative charge, and that surround the nucleus of an atom. When a bunch of electrons all move together in the same direction, they form an electric current.

Electrons in an atom can be thought of as being somewhere in an array of possible “states” — which include their energy level, momentum and spin — with different probabilities of being in a given state. Two electrons can’t be in the same state at the same time — that is, at least one of these variables must differ. Some particular states are possible, and some are forbidden by the laws of . Sets of possible states form regions that are called bands. Sets of states that are not possible form regions between those bands, and these are called bandgaps.

The bands closest to the , called core levels, and the furthest band from the nucleus that has electrons in it, called the valence band, all keep their electrons tightly in place. The next band out from that is called the “,” and there, the electrons are free to roam around freely.

In some materials, called metals, a valence band and a conduction band overlap, and electricity flows freely and easily through them. In other materials, called insulators, there is a wide gap between the valence band and the conduction band, making it almost impossible for an electron to get excited enough to jump from one to the other, so they block the flow of electricity.

There’s a third category, and that’s where the most interesting stuff happens. These are materials that have a narrower gap between the two bands, and they are called semiconductors. Sometimes they can act like metals, sometimes they can act like insulators, and sometimes they can have properties in between. When first discovered, they were considered useless because of their erratic, variable behavior — until physicists figured out the mystery of the bandgap.

“It was the idea of the bandgap that allowed people to understand and harness semiconductors for optoelectronic devices” — that is, devices that work with light and electricity — says Tonio Buonassisi, MIT’s SMA Assistant Professor of Mechanical Engineering and Manufacturing.

When electrons get excited (by getting heated, or by being hit with a particle of light, known as a photon), they can jump across the gap. If an electron in a crystal gets hit by a photon that has enough energy, it can get excited enough to jump from the valence band to the conduction band, where it is free to form part of an electric current. That’s what happens when light strikes a solar cell, producing a flow of electrons.

Silicon, a semiconductor, is the material of choice for in large part because of its bandgap. Silicon’s bandgap is just wide enough so that electrons can easily cross it once they are hit by photons of visible light.

The same process also works in reverse. When electricity passes through a semiconductor, it can emit a photon, whose color is determined by the material’s bandgap. That’s the basis for light-emitting diodes, which are increasingly being used for displays and computer screens, and are seen as the ultimate low-power light bulbs.

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More information: Video: "Next Generation Solar Cells" with Tonio Buonassisi - mitworld.mit.edu/video/675
More "Explained" stories: www.physorg.com/search/sort/da… xplained:&headline=1

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User comments : 11

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Bob_Kob
Jul 23, 2010
This comment has been removed by a moderator.
snivvy
not rated yet Jul 23, 2010
Yes it does! Not as efficiently as a device designed to generate power but the reaction is indeed bidirectional. For example, if you had an LED light source that you wanted to turn on automatically, you could 'read' one or more of the LEDs to sense if the ambient light exceeded a predetermined threshold, then turn the power on to the LEDs to provide illumination. On a periodic basis, for example, on millisecond each minute, you could 'read' the ambient light level, then turn the current back on if the sensor said it was 'dark'.
Jarek
3.7 / 5 (3) Jul 23, 2010
Quantum mechanics says that bandgaps have to be exceeded for electron jump, that stationary probability of electrons should be the ground state like on these recent STM pictures of semiconductor's defected lattice of potential wells:
http://physicswor...ws/41659
... but doesn't say much about current flow behind - stochastic dynamics of electrons.
To understand it there could be useful new models: based on Maximal Entropy Random Walk - they give the same stationary probability density as QM and to start conductance we have to exceed entropic barriers - here is fresh demonstration about it:
http://demonstrat...ndomWalk
croghan27
not rated yet Jul 23, 2010
So shining light on an LED will create a current?


If you have a battery powered LED (such as a pointer) can you recharge the battery by shining a light on it?
Jigga
2 / 5 (4) Jul 23, 2010
If you have a battery powered LED can you recharge the battery by shining a light on it...
Electrode interface of batteries is often behaving like pair of serial diodes, too - and some of them are quite reversible. So theoretically yes - practically not.
maxcypher
not rated yet Jul 23, 2010
Jarek -- I have to thank you for the comments and the two links you provided. Extremely useful.
jbarnhill1
not rated yet Jul 24, 2010
I read this article while listening to The Gap Band. I think it actually made it more informative.
doctore2010
not rated yet Jul 28, 2010
somebody knows what really happening inside of material? which of two coupled electrons is going to be free?thx
Jigga
Jul 28, 2010
This comment has been removed by a moderator.
doctore2010
not rated yet Jul 28, 2010
it is about solar cell. Each solar cell consist of covalent bonds of (two) electrons.My questions was which of them is going to be free after the photon absorption?
Jigga
1 / 5 (3) Jul 28, 2010
Each solar cell consist of covalent bonds of (two) electrons.
It doesn't. Problem solved.
doctore2010
not rated yet Jul 29, 2010
what do you mean? ``it doesn´t``
that is Bandgapmodel about. Bandgap is energy needed to extract those 2 (spin-bounded) electrons. The bonds are stronger than coulomb force, which take place after the spin-bonds are gone.
Jigga
2.7 / 5 (3) Jul 29, 2010
.. Bandgap is energy needed to extract those 2 (spin-bounded) electrons....

So far I believed, band gap models describes the destiny of electrons and holes - not pairs of electrons.

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